Many different kinds of multimode scanning and detection systems are currently known. Such systems may be active or passive in operation, being operationally effective in scanning or detecting multiple beams of radiation at multiple frequencies and wavelengths. The frequencies of operation include infrared radiation, in which heat is detected to identify a particular target or target region. Detection may be accomplished in the radar or radio frequency bands, either actively or passively or subject to a combination of active and passive modes.
The term multimode can further be taken to refer to detection first at one mode of energy operating at a given first frequency, and then detection at another selected mode or frequency. When several frequencies of the electromagnetic spectrum are thereby used, this approach is frequently referred to as multi-spectral. Multimode can further be taken to mean the use of both active and passive bands of radiation. It can additionally mean the use of one or more radar bands of radiation and one or more infrared bands. Multimode detection systems can moreover be ground based, ship based, airborne or set aloft in space.
In general, multimode detection systems enhance the detection flexibility and effectiveness of the system using the technique. For example, one beam may be designed to be wide in shape in order to conduct search operations for a target sought, and the other beam working in conjunction therewith is then narrow in order to accomplish tracking once the target has been identified. The different modes can relate to the distance or range of detection as well. For example, one mode can be used for short range target acquisition, while the other mode is employed at more extended ranges. For example, radar frequencies might be used at long ranges and infrared frequencies closer in.
The various modes of operating such detection systems can moreover be used in combination with each other in order to accomplish effective target classification and identification. For example, targets often appear different in different spectral regions, and the degree of difference can be used to distinguish one type of target from another.
As desirable as multimode systems may be, problems nonetheless arise in the development of multimode systems due to the relationships between the modes. For example, techniques and arrangements have been urgently needed to establish coordination between the modes of radiation selected, to permit effective handoff between the modes of operation to ensure a continuity of information and operation. Other problems faced in implementing multimode systems are caused by the limited nature of refractive materials available for use as protective domes, collimating lenses, and the scanning system itself, in order to permit unhampered egress and ingress of the selected beams of radiation to be scanned or detected.
The prior art often achieves beam scanning by mechanical pointing means, for example, by mounting entire antenna systems on gimbals. Such methods are more costly, cumbersome and prone to breakdown than the rotating refractive prism scanners according to the invention herein.
Other difficulties arise in designing an effective multimode scanning arrangement with rotating prisms when the beams scanned are at different frequencies, because beams of different frequencies typically are not deviated by the same amplitude. This not only causes such beams to point in different directions from time to time, but it also causes the difference in these directions to change by an amount which depends upon the pointing direction, thereby hampering transfer from one mode of operation to the other. In other words, because the same scanning prisms are utilized for both beams, handoff from one mode to the other becomes more difficult to accompiish.